Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for horn antennas, and more particularly to horn antennas which can operate in multiple frequency bands.
Description of the Related Art
Conventional electromagnetic waveguides and horn antennas are well known in the art. A waveguide is a transmission line structure that is commonly used for microwave signals. A waveguide typically includes a material medium that confines and guides a propagating electromagnetic wave. In the microwave regime, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid, liquid, liquid crystal or gaseous dielectric material.
In a waveguide, a “mode” is one of the various possible patterns of propagating or standing electromagnetic fields. Each mode is characterized by frequency, polarization, electric field strength, and magnetic field strength. The electromagnetic field pattern of a mode depends on the frequency, refractive indices or dielectric constants and relative permeabilities, and waveguide or cavity geometry. With low enough frequencies for a given structure, no transverse electric (TE) or transverse magnetic (TM) modes will be supported. At higher frequencies, higher modes are supported and will tend to limit the operational bandwidth of a waveguide. Each waveguide configuration can form different transverse electric or transverse magnetic modes of operation. The most useful mode of propagation is called the Dominant Mode. Other modes with different field configurations can occur unintentionally or can be caused deliberately.
In operation, a waveguide will have field components in the x, y, and z directions. A rectangular waveguide will typically have waveguide dimensions of width, height and length represented by a, b, and l respectively. The cutoff frequency or cutoff wavelength (for transverse electric (TE) modes) for a rectangular waveguide can be represented as:             (              f        c            )        mn    =                    1                  2          ⁢          π          ⁢                      μɛ                              ⁢                                                  (                                                m                  ⁢                                                                           ⁢                  π                                a                            )                        2                    +                                    (                                                n                  ⁢                                                                           ⁢                  π                                b                            )                        2                              ⁢                           ⁢                        and          ⁢                                          (                      λ            c                    )                mn              =          2                                                  (                              m                a                            )                        2                    +                                    (                              n                b                            )                        2                              where a is the width of the wider side of the waveguide, and b is a width of the waveguide measured along the narrow side, ε and μ are the permittivity and permeability of the dielectric inside the waveguide, and m, n are mode numbers. The lowest frequency mode in a rectangular waveguide is the TE10 mode. In this mode, the equation for the signal wavelength at the cutoff frequency reduces to λc=2a. Since waveguides are generally designed to have a static geometry, the operational frequency and bandwidth of conventional waveguides is limited.
Horn antennas are essentially open-ended waveguides in which the walls are gradually flared outwardly toward the radiating aperture. Horn antennas can be designed to support a particular mode, depending on the desired RF propagation antenna radiation pattern.
A type of horn antenna is a corrugated horn antenna. A corrugated horn antenna typically includes circumferential grooves, or corrugations, along the interior walls of the antenna. The depth of the corrugations are typically approximately one-quarter of a wavelength at the operating frequency, which substantially increases the surface impedance of the wall as compared to a smooth wall. The increased surface impedance results in the corrugated horn antenna having a symmetrical radiation pattern, that is, equal magnetic field and electric field radiation pattern plane cuts. The dominant mode in the corrugated conical horn is the HE11 mode. In the HE11 mode the corrugated horn has greater bandwidth as compared to a horn antenna having smooth walls and the corrugated horn exhibits lower attenuation than any mode of a horn antenna of comparable size. Nonetheless, the operational bandwidth of a typical corrugated horn antenna is still less than one octave.
To overcome the frequency and bandwidth limitations of horn antennas, International Patent Application No. PCT/GB92/01173 assigned to Loughborough University of Technology (Loughborough) proposes that a frequency selective surface (FSS) can be used within a waveguide to influence the frequency response. An FSS is typically provided in one of two arrangements. In a first arrangement, two or more layers of conductive elements are separated by a dielectric substrate. The elements are selected to resonate at a particular frequency at which the FSS will become reflective. The distance between the layers of conducive elements is selected to create a bandpass condition at a fundamental frequency at which the FSS becomes transparent and passes a signal. The FSS also can pass harmonics of the fundamental frequency. For example, if the fundamental frequency is 10 GHz, the FSS can pass 20 GHz, 30 GHz, 40 GHz, and so on.
Alternatively, FSS elements can be apertures in a conductive surface. The dimensions of the apertures can be selected so that the apertures resonate at a particular frequency. In this arrangement, the FSS elements pass signals propagating at the resonant frequency. Any other electromagnetic waves incident on the FSS surface are reflected from the surface.
In a multi-band waveguide or horn antenna, the FSS can form a second horn within a first horn wherein the second horn and first horn are tuned to different frequencies. This concept is not without its drawbacks, however. In particular, the horn proposed by Loughborough can generate grating lobes, which is electromagnetic energy that is scattered to uncontrolled directions. Grating lobes result from transmitted and scattered plane waves which do not obey Snell's laws of reflection and refraction. Causes of grating lobes are relatively large inter-element spacing within the FSS, large angles of incidence of plane wave with respect to surface, and/or both. Importantly, grating lobes adversely effect horn antenna performance and should be avoided.
Further, the walls of the horns proposed by Loughborough consist of conventional FSS's. Notably, Loughborough's horns do not include corrugations on the horn walls. Such corrugations would disrupt the transparency of the conventional FSS's. Specifically, conventional FSS elements are rather large on comparison to the distance between corrugation ridges. The separation between corrugation ridges may be less than a diameter of a conventional FSS element. Thus, the corrugation ridges would overlap the FSS elements and disrupt FSS element operation, thereby severely degrading the performance of the horns. Accordingly, there exists a need for a corrugated horn antenna incorporating a FSS, wherein the corrugations do not disrupt operation of the FSS elements.